The asphalt used for paving refining and industrial uses is a solid or semi-solid bituminous material that is either naturally occurring, or derived from petroleum refining processes and includes paraffinic and aromatic hydrocarbons and heterocyclic compounds. In paving, mineral aggregates such as crushed stone are typically mixed with asphalt materials, producing pavement-type products suitable for vehicular or related traffic, such as those seen in Table 1. In addition to asphalt use in road and highway applications, asphalt is a commonly used material for construction purposes, such as roofing materials, water and damp-proofing products, bridge decks, racetracks, airport runways, parking lots, bicycle paths, and port facilities.
TABLE 1Typical Constituents of Asphalt Concrete Used inPavement Applications.Type of ConstituentExamplesAggregate Materialmineral aggregates, crushed concrete, flyash, sand, gravel, crushed stone, slags,screenings, recycled asphalt pavement,recycled asphalt shinglesBindersbitumen/asphaltAdditivescellulose fibers, synthetic mats and grids
The performance required of any asphalt material is determined by its end use and/or application and is gauged by one or more measurable properties.
The most common type of paving composition in the United States is hot mix asphalt (HMA), which involves melting a pre-selected mixture of mineral aggregate and asphalt in a large heated tank. There are numerous mix designs used to meet climatic conditions and vehicular traffic. The mineral aggregate particle size gradation and asphalt type providing the optimum set of performance properties is referred to as the Job Mix Formula, of which there are numerous variations to satisfy the requirements of the pavement to include: vehicular traffic, climate conditions, and useful life expectancy.
Some major problems associated with the performance of hot mix asphalts (HMA) pavements can be moisture susceptibility (striping), permanent deformation (rutting), bleeding, shoving, and cracking (thermal and fatigue). The asphalt binder is selected for the paving environment to provide sufficient stiffness to resist rutting at expected high service temperatures and enough flexibility to resist fatigue and thermal cracking at intermediate and low service temperatures. Asphalts are exposed to weathering, which provokes aging due to a decrease in the maltenes phase, produced by oxidation. This asphalt aging is dependent upon the temperature and partial pressure to which the asphalt is exposed (Glover, et al., Evaluation of binder aging and its influence in aging of hot mix asphalt concrete: literature review and experimental design. (Texas Transportation Institute, 2009); chapter 2). The oxidation forms carbonyl (COO) functional groups which cause the asphalt to stiffen, becoming less elastic and viscous. As the asphalt stiffens, fatigue resistance decreases significantly, resulting in cracking and loss of structural integrity (Glover, et al., Evaluation of binder aging and its influence in aging of hot mix asphalt concrete: literature review and experimental design. (Texas Transportation Institute, 2009); chapter 2).
Once the asphalt loses structural integrity, it must be resurfaced. Resurfacing is typically performed using in situ processing of the asphalt, such as hot in-place asphalt processing, or repaving. However, those skilled in the art use a blend of virgin asphalt with existing asphalt, called Reclaimed Asphalt Pavement (RAP). For example, hot in-place asphalt processing heats the damaged pavement to 107° C. at a depth of 20 to 40 mm (¾ to 1½ in) using propane burners, followed by scarifying the pavement and blending virgin aggregate and compacted (Kandhal & Mallick, Pavement Recycling Guidelines for State and Local Governments Participant's Reference Book. (U.S. Dept of Transportation 1997); chapter 11).
Alternatively, the road can be milled and repaved. It is known that the milling of old road surfaces provides a number of advantages in preparing the old roadbed for resurfacing. Milling uses rotating drums possessing cutters disposed around the outer circumference, typically rotating opposite to the direction of drive wheels, to remove the upper layer of asphalt. It not only ensures a new, smooth and level base for the new hot mix overlay, but at the same time lowers the road bed height to maintain bridge deck clearances and curb and gutter depths. Grinding or milling is also beneficial in removing potholes, old cracks, joint seams, and ruts along with other surface damage that would quickly reappear in a new surface overlay if not repaired. The milled asphalt is collected and transported to a holding bin, and subsequently transported to a storage location or disposal landfill.
Generally, asphalt-paved highways, driveways, avenues, and streets are recyclable. This is because asphalts are mainly composed of asphaltenes and maltenes, which can be oxidized as discussed above. The majority of existing roadways, both concrete and bituminous asphalt, undergo constant repair and surface overlay with new hot mix asphalt to achieve and maintain safe and comfortable high speed riding surfaces. In most instances the new asphalt does not contain purely virgin asphalt, but includes a percentage of recycled asphalt, taken from road milling and processed for reuse. Reclaimed Asphalt Pavement (RAP) includes removed and/or reprocessed pavement materials containing asphalt and aggregates. These materials are generated when asphalt pavements are removed for reconstruction, resurfacing, or to obtain access to buried utilities. After removing the damaged asphalt from roads, asphalt recycling plants are used for recycling. Typically, asphalt recycling plants average 400 tons per hour to 600 tons per hour production ranges and 15% to 30% RAP can be injected into these plants.
To recycle asphalt, those of skill in the art add the RAP directly to virgin asphalt. The virgin asphalt is superheated, as seen in Table 2, followed by mixing the RAP and virgin asphalt together. The virgin asphalt transfers heat to the RAP during mixing, to provide adequate rheological properties for mixing and laying the roadway. However, during this process aggregates are heated above 650° F., which is the flash point for asphalt cement, and added to the asphalt cement, thereby transferring heat from the aggregate. The superheating causes material formation on the dryer, hot elevator and screen tower, requiring extensive clean up or replacement of those parts. More importantly, the heat transfer from the superheated material results in release of maltenes from the asphalt, as seen by the excessive smoke formation, and causes premature aging of the virgin asphalt, i.e. rapid oxidation due to the high temperatures.
TABLE 2Temperature Requirements for Virgin Asphalt in for RAP-Virgin asphalt mixingto meet desired RAP and moisture content (taken from Kandhal & Mallick, PavementRecycling Guidelines for State and Local Governments Participant's Reference Book.(U.S. Dept of Transportation 1997); chapter 5).RecycledWaterVirgin asphalt temperature to achieve recycled mix dischargeasphaltcontenttemperature of:composition(%)104° C. (220° F.)115° C. (240° F.)167° C. (260° F.)138° C. (280° F.)10% RAP/0121° C. (250° F.)138° C. (280° F.)152° C. (305° F.)163° C. (325° F.)90% virgin1127° C. (260° F.)143° C. (290° F.)154° C. (310° F.)168° C. (335° F.)3138° C. (280° F.)149° C. (300° F.)163° C. (325° F.)174° C. (345° F.)5193° C. (290° F.)157° C. (315° F.)168° C. (335° F.)182° C. (360° F.)30% RAP/0157° C. (315° F.)179° C. (345° F.)191° C. (375° F.)207° C. (405° F.)70% virgin1168° C. (335° F.)185° C. (365° F.)202° C. (395° F.)218° C. (425° F.)3196° C. (385° F.)213° C. (415° F.)229° C. (445° F.)246° C. (475° F.)5224° C. (435° F.)241° C. (465° F.)257° C. (495° F.)274° C. (525° F.)50% RAP/0210° C. (410° F.)235° C. (455° F.)257° C. (495° F.)282° C. (540° F.)50% virgin1240° C. (465° F.)268° C. (515° F.)288° C. (550° F.)310° C. (590° F.)3302° C. (575° F.)327° C. (620° F.)349° C. (660° F.)374° C. (705° F.)5365° C. (690° F.)390° C. (735° F.)413° C. (775° F.)438° C. (820° F.)
While the temperatures in Table 2 have been deemed appropriate to achieve the desired mix temperature at the exit, temperatures are higher due to the recycling system design. For example, when cold wet RAP is injected into hot mix plants, the super-heated virgin aggregates (600° to 900° Fahrenheit) conductively transfer enough heat to the cold wet RAP for drying and heating all materials to a mixing temperature of 300° F. However, it is common that the sudden and violent steam expansion that is created when the super-hot aggregate (600° to 900° Fahrenheit) encounters the cold wet RAP instantly overloads exhaust system airflow capacity and results in fracturing of the asphalt. Such fracturing results in the asphalt mix gradations drifting out of specifications. Further, this superheating process leads to excessively high temperatures to dry and heat RAP and can therefore inflict heat damage, premature wear, and failure to the virgin asphalt. Another problem is that Plant productive capacity drops off dramatically when running RAP due to exhaust system and dryer burner overloads from RAP steam blockages within dryers.
Moreover, rapid heating of the asphalt material to heat too abruptly causes surges of steam that damage the recycling system, and also degrade the structural integrity of the asphalt cement.
While current methods provide for low-concentrated use of reclaimed asphalt, resulting asphalt mixes typically possess comprised performance with respect to wear. As such, there is an unmet need to provide novel processing systems to rejuvenate reclaimed asphalt that possess aging and temperature tolerance properties that mimic virgin asphalt.